The stepper is the most expensive equipment used in the fabrication of an integrated circuit (IC). Therefore, increasing the throughput of the stepper is always considered as a key factor to improve the fabrication process of the integrated circuit. In order to use the throughput of the stepper efficiently, the wafer foundry does their best to design patterns within the available area of the mask so as to reduce the exposure frequency and exposure time.
For 0.13 μm generation process, the development cost of a set of masks has increased to about $1.0 million, which is too much to be affordable for a middle or small IC design house. Therefore, a wafer foundry in Taiwan, “Taiwan Semiconductor Manufacturing Company” has developed a solution called “Cyber Shuttle” or “Multi-Project Mask” to reduce the cost of developing a mask by the IC design house. The Cyber Shuttle divides each mask into several blocks, and different IC design houses will then subscribe required blocks and share the total design and fabrication cost of the mask. Consequently, the shared design and fabrication cost of the mask for each IC design house will be decreased. However, the Cyber Shuttle is only suitable for the integrated circuit to be fabricated by the process of the same generation and with a high compatibility, and an IC design house using a leading fabrication process may not find another IC design house using the same generation fabrication process to share the cost of the mask.
In addition, the IC design house may modify the circuit layout several times during the development of the die to verify the electrical property and qualification, and each modification of the circuit layout needs a new mask. Generally speaking, about 3 to 5 sets of masks are required to complete the development of the die, which results in the cost of the mask representing the majority of the total cost of the die. Particularly, this business model is feasible for fabricating a large number of IC dies since multiple IC dies can share the expensive fabrication cost of the mask equally. However, diversified IC dies with small quantity requirements are not suitable for the above-mentioned business model since fabrication costs of the mask shared by each die are relatively high. Therefore, further decreasing the cost of the mask, which is also decreases the cost of the die and increases the competitiveness of IC design house, is an important objective.